A denitrification technology with microorganisms has conventionally been performed with water containing ammonia nitrogen, for example, sewage, as a target. In this biological nitrogen removal technology, ammonia nitrogen in water to be treated is converted into a nitrogen gas through a two-stage biological reaction, that is, nitrification and denitrification and then, the resulting nitrogen gas is discharged out of the system. Described specifically, in the nitrification step, ammonia nitrogen is oxidized into nitrite nitrogen by making use of oxygen attributable to ammonia oxidizing bacteria under aerobic conditions and then the nitrite nitrogen is oxidized into nitrate nitrogen with nitrite oxidizing bacteria. Then, in the denitrification step, the nitrite nitrogen and the nitrate nitrogen are converted into a nitrogen gas by using denitrifying bacteria under oxygen free conditions while using an organic matter as an electron donor.
Such a biological nitrogen removal technology requires a large amount of oxygen in the nitrification step, while a large amount of an organic matter such as methanol should be added in the denitrification step, which totally increases a running cost. As a new biological nitrogen removal technology which has succeeded in overcoming such a technological problem, a biological nitrogen removal method and a biological nitrogen removal device making use of an anaerobic ammonia oxidation reaction are used. The term “anaerobic ammonia oxidation reaction” as used herein means a biological reaction making use of anaerobic ammonia oxidizing bacteria. The anaerobic ammonia oxidizing bacteria belong to a denitrification microorganism group capable of reacting ammonia nitrogen as an electron donor with nitrite nitrogen as an electron acceptor under anaerobic conditions to produce a nitrogen gas and they are denitrifying microorganisms which do not require addition of an organic matter upon denitrification.
Patent Document 1 discloses an example of a biological nitrogen removal method and a biological nitrogen removal device making use of such an anaerobic ammonia oxidation reaction. These biological nitrogen removal method and biological nitrogen removal device are roughly comprised of, from the upstream side to the downstream side of water to be treated, a partial nitritation tank, a pH regulating tank, and a denitrification tank. More specifically, in the partial nitritation tank, an ammonia nitrogen containing solution is aerated in the presence of ammonia oxidizing bacteria and a portion of an ammonia nitrogen component is oxidized even to nitrite nitrogen. In the denitrification tank, nitrite nitrogen and ammonia nitrogen in the water to be treated in the partial nitritation tank are reacted with each other to convert them into a nitrogen gas in the presence of anaerobic ammonia oxidizing bacteria. In the pH regulating tank, the water to be treated in the denitrification tank is circulated to the partial nitritation tank to effect pH regulation. A biological nitrogen removal method and a biological nitrogen removal device having such a constitution make it possible to carry out nitrification at a reduced aeration power and at the same time, omit addition of an organic matter such as methanol and thereby reduce a production amount of a sludge.
In such a biological nitrogen removal method and a biological nitrogen removal device, the partial nitritation tank, the pH regulating tank, and the denitrification tank should be provided separately and the pH in the nitritation tank and the denitrification tank should be adjusted to a pH value necessary for the partial nitritation reaction and the denitrification reaction, respectively, so that the device needs a high equipment cost as a biological nitrogen removal device and the method is not simple as a biological nitrogen removal method.
A biological nitrogen removal device and a biological nitrogen removal method making use of an anaerobic ammonia oxidation reaction, which have solved the above technological problem, are disclosed, for example, in Non-Patent Document 1. In this biological nitrogen removal device, nitrogen is removed by using a partial nitritation tank and a denitrification tank commonly as a single tank and causing a partial nitritation reaction and a denitrification reaction in this single tank without regulating a pH value. More specifically, a single tank into which water to be treated has been poured is charged with a support having, on the surface portion thereof, a two-layered microbial film having, in the outer layer, nitrite type nitrifying bacteria contributing to a nitrite type nitrification reaction as a dominant species and, in the inner layer, anaerobic ammonia oxidizing bacteria as a dominant species and the support is caused to flow in the water to be treated containing ammonia nitrogen to oxidize a portion of ammonia nitrogen into nitrite nitrogen through a nitrite type nitrification reaction with nitrite type nitrifying bacteria under aerobic conditions. Then, under anaerobic conditions in which presence of the nitrite type nitrifying bacteria enables blocking from oxygen in the water to be treated, the nitrite nitrogen and ammonia nitrogen are reacted and converted into a nitrogen gas through the action of the anaerobic ammonia oxidizing bacteria mainly present in the inner layer of the microbial film. Thus, denitrification is conducted. The biological nitrogen removal device and biological nitrogen removal method making use of such an anaerobic ammonia oxidation reaction make it possible to simplify the equipment and thereby reduce a cost and at the same time, omit the regulation of a pH value and thereby achieve simplification of the biological nitrogen removal method.
This biological nitrogen removal method making use of the anaerobic ammonia oxidation reaction however has the following technological problem. Described specifically, the water to be treated containing ammonia nitrogen cannot be subjected to biological nitrogen removal freely under any conditions because there are limitations on the ammonia nitrogen concentration of the water to be treated, which is a target of the treatment, and water temperature, DO value, and pH value in the reaction tank.
More specifically, according to the conventional biological nitrogen removal method making use of an anaerobic ammonia oxidation reaction, water to be treated containing ammonia nitrogen can be denitrified by using the following reaction process.
(1) nitrite type nitrification reaction:NH4++1.5O2→NO2−+H2O+2H+
(2) anaerobic ammonia oxidation reaction:0.75NH4++NO2−→0.77N2+0.19NO3−+1.5H2O+0.10H−
More specifically, nitrous acid is produced by placing, under aerobic conditions in the water to be treated containing ammonia nitrogen, a support having, on the surface portion thereof, a two-layered microbial film having, in the outer layer, nitrite type nitrifying bacteria contributing to a nitrite type nitrification reaction as a dominant species and, in the inner layer, anaerobic ammonia oxidizing bacteria contributing to an anaerobic ammonia oxidation reaction as a dominant species while being surrounded with the nitrite type nitrifying bacteria and thereby causing the nitrite type nitrification reaction, which is the reaction of (1), by the action of the nitrite type nitrifying bacteria present in the outer layer as a dominant species. Then, based on the ammonia nitrogen in the water to be treated and nitrate nitrogen thus produced, an anaerobic ammonia oxidation reaction which is the reaction of (2) is caused by the anaerobic ammonia oxidizing bacteria while satisfying anaerobic conditions because the anaerobic ammonia oxidizing bacteria are present in the inner layer of the microbial film while being surrounded with the nitrite type nitrifying bacteria, leading to the formation of nitrogen.
During the denitrification method by making use of such an anaerobic ammonia oxidation reaction, however, nitrous acid thus produced is inevitably oxidized into nitric acid through the nitrite type nitrifying bacteria as shown in the following reaction formula:
(3) Nitrate type nitrification reaction:NO2−+0.5O2→NO3−+H2O+2H+
In order to perform a biological nitrogen removal method at a practical level by making use of an anaerobic ammonia oxidation reaction, it is necessary to inhibit the nitrate type nitrification reaction, which is the reaction of (3), while increasing the amount of nitrous acid produced using the nitrite type nitrification reaction which is the reaction of (1), and thereby secure ammonia nitrogen and nitrite nitrogen necessary for the anaerobic ammonia oxidation reaction which is the reaction of (2).
In this respect, parameter factors such as ammonia nitrogen concentration in water to be treated and water temperature, DO value, and pH value in the tank are presumed to have an influence on the nitrite type nitrification reaction and the nitrate type nitrification reaction.
FIGS. 8 to 11 are graphs schematically showing the influence of a temperature of water to be treated, a DO (dissolved oxygen content) in the water to be treated, an ammonia nitrogen concentration in the water to be treated, and a pH in the water to be treated, respectively, on a proliferation rate or a reaction rate of nitrite type nitrifying bacteria and nitrate type nitrifying bacteria. As shown in FIG. 8, the higher the temperature of water to be treated, the higher the proliferation rate of the nitrite type nitrifying bacteria compared with that of the nitrate type nitrifying bacteria. The water to be treated therefore has preferably a high temperature. As shown in FIG. 9, the lower the DO (dissolved oxygen content) in the water to be treated, the lower the reaction rate of the nitrate type nitrifying bacteria compared with that of the nitrite type nitrifying bacteria. The water to be treated therefore has preferably a lower DO value while satisfying aerobic conditions. Further, as shown in FIG. 10, the higher the ammonia nitrogen concentration in the water to be treated, the lower the reaction rate of the nitrate type nitrifying bacteria compared with that of the nitrite type nitrifying bacteria. The water to be treated has preferably a higher ammonia nitrogen concentration. Furthermore, as shown in FIG. 11, the higher the pH of the water to be treated, the lower the proliferation rate of the nitrate type nitrifying bacteria compared with that of the nitrite type nitrifying bacteria. The water to be treated has therefore preferably a higher pH value.
As described above, in order to inhibit a nitrate type nitrification reaction, which is the reaction of (3), while increasing the amount of nitrous acid produced using a nitrite type nitrification reaction which is the reaction of (1), there are limitations on the ammonia nitrogen concentration in the water to be treated and the water temperature, DO value, and pH value in the tank. Thus, it is difficult to say that denitrification of water to be treated containing ammonia nitrogen can be performed at a practical level under any conditions by using an anaerobic ammonia oxidation reaction. This method is applied only to the treatment of wastewater, such as industrial wastewater and returned water from sludge treatment, having a high temperature and a high ammonia nitrogen concentration. Since municipal wastewater or domestic wastewater has a lower temperature and a lower ammonia nitrogen concentration, it is difficult to apply the method to the treatment of such wastewater as is.